1. Field of the Invention
This invention relates to an ink feeding method and ink feeding apparatus for a printing machine, for controlling an ink feeding rate by measuring color density of prints produced.
2. Description of the Related Art
Such a printing machine has an ink feeding apparatus for adjusting the rate of feeding ink to ink rollers. The ink feeding apparatus includes a plurality of ink keys juxta-posed in a direction perpendicular to a direction in which printing paper is transported in time of printing. The rate of feeding ink to the ink rollers is adjusted by varying the opening degree of each ink key. In this way, the rate of feeding ink ultimately to a printing plate is adjusted.
The printing plate has areas called detecting patches or control strips formed in positions corresponding to the respective ink keys. The color density of the detecting patches actually printed on the printing paper is measured with a densitometer to adjust the opening degree of each ink key.
When printing with such a printing machine, the color density of prints may not agree with a predetermined value immediately after start of a printing operation even though the ink keys in the ink feeding apparatus have a proper opening degree. In such a case, when the color density of prints is measured and the ink feeding rate is automatically controlled, the opening degree of the ink keys, even though proper, is further adjusted in an opening direction.
Since numerous ink rollers are used in such a printing machine, a predetermined time is taken until an adjustment of the opening degree of each ink key is reflected in the rate of feeding ink to printing paper. Thus, when the ink feeding rate is automatically controlled by measuring the color density of prints immediately after adjusting the opening degree of the ink keys, the opening degree of the ink keys is further adjusted even though the opening degree is proper.
The rate of feeding dampening water to the printing plate influences the rate of feeding ink to the printing plate. Thus, when the ink feeding rate is automatically controlled by measuring the color density of prints immediately after adjusting the rate of feeding dampening water to the printing plate, the opening degree of the ink keys is further adjusted even though the opening degree is proper.
An adjustment of the opening degree of the ink keys, therefore, is prohibited immediately after start of a printing operation, or after an adjustment is made of the ink or water feeding rate, until a predetermined number of sheets are printed or until lapse of a fixed time.
However, where a long time is set for the above prohibition, the ink feeding rate cannot be controlled quickly. This presents a problem of taking a long time before the color density of actual prints settles at a target value.
On the other hand, when the opening degree of the ink keys is varied excessively to control the ink feeding rate quickly, a gross overshooting will occur before the color density of prints settles at a target value.